The rate equation describing the time course of enzymic reactions in the presence of tight-binding inhibitor has been derived. Several experimental designs and statistical methods have been developed. 1) Attempts will be made to modify the rate equation so that it may describe the time courses of inhibited enzymic reactions when the enzyme and/or the enzyme-inhibitor complex undergoes conformational changes. 2) The above methods will be applied to dihydrofolate reductases from L. casei and E. coli inhibited by methotrexate. Dissociation constants of various enzyme-substrate complexes (E.FH2, E.NADPH, E.MTX.NADPH) and enzyme-inhibitor complexes (E.MTX, ENADPH.MTX) as well as the first and second order rate constants for the enzyme-inhibitor interactions are expected to be determined.